1. Field of the Invention
This invention relates to a method for controlling a welding power source, and more particularly to a method for controlling the output of a power source in short circuit welding in which a short-circuiting phase and an arcing phase alternately take place between a consumable electrode and a base metal.
2. Description of the Prior Art
Shown in FIGS. 1(a)-(g) are sequential stages through which a molten drop is formed and transferred in one cycle of a consumable electrode welding process with alternate short circuiting and arcing phases. In these FIGURES, indicated by 1 is a consumable electrode (hereinafter referred to simply as "welding wire" for brevity), by 2 a molten drop formed at the tip end of the welding wire 1, by 3 an arc, and by 4 a weld pool or workpiece. Further, shown in FIG. 1(a) is an initial stage of short circuiting in which the molten drop 2 begins to contact the weld pool 4, in FIG. 1(b) a middle stage of the short circuiting in which the molten drop 2 is in full contact with and being transferred to the weld pool 4, in FIG. 1(c) a final stage of the short circuiting in which necking occurs between the welding wire 1 and molten pool 4 as a result of transfer of the molten drop 2 to the weld pool 4, in FIG. 1(d) the instant of restriking an arc 3, in FIG. 1(e) an initial arcing stage in which the tip end of the welding wire 1 is melted, forming a growing molten drop 2 thereat, and in FIGS. 1(f) and (g) arcing stages immediately before short circuiting by the molten drop 2 which is about to contact the weld pool 4. The stages of FIGS. 1(a) to 1(g) are repeated in the welding operation.
Referring to FIG. 2, there are shown waveforms of welding current and voltage of a conventional welding power source which has constant potential characteristics by combined use of a reactor. Reference characters (a) to (g) which indicate particular points of the waveforms in FIG. 2 correspond to the molten drop forming and transferring stages in FIGS. 1(a) to 1(g), respectively.
With such a conventional welding power source, there often arise various problems as follows. Namely, in the stage in FIG. 1(a), the welding current begins to increase with a certain time constant immediately after short circuiting between the molten drop 2 and weld pool 4, and, if the sectional area of a contact portion A of the molten drop and weld pool 4 is small, that is to say, if the welding current becomes larger before the transfer to the weld pool 4 of the molten drop 2, the short circuit is ruptured and an arc is generated, causing spattering. In the stages of FIGS. 1(c) and 1(d), necking occurs to the molten drop 2 and the short circuit is ruptured to generate an arc again. The welding current reaches a highest value at this re-arcing time, and a large amount of splattering is produced by repulsive energy of the arc which shakes the weld pool 4. In the stages of FIGS. 1(d) and 1(e) which follows the onset of the arc, the welding current increases when the preceding short circuit period is long and decreases when preceded by a long short circuit period. Accordingly, the size of the molten drop 2 which is formed in the stages of FIGS. 1(d), 1(e) and 1(f) becomes irregular, and, if it is too small, an unfused portion of the welding wire 1 is stuck into the weld pool 4 in the stage shown in FIG. 1(g), putting the welding operation in a considerably unstable state. Further, the welding current should be small in the stage shown in FIG. 1(g) in order to urge the molten drop 2 toward the weld pool to encourage the short circuiting. However, in these stages the welding current decreases toward the current I=E/R according to the inductance L, output voltage E and equivalent resistance R in the circuit, in proportion to the value e(-L/R.sup.t) as shown in FIG. 2. Therefore, where the welding voltage has a high mean value, namely, where the welding current has a high mean value, large current flows in the stage of FIG. 1(g) to disencourage the short circuiting. Besides, as a welding power source has constant potential characteristics, reductions in arc length are reflected by current increases. Consequently, short circuiting becomes more difficult and the molten drop grows into a large size, resulting not only in irregular short circuit periods but also in splattering of larger droplets.
In this connection, reference is also made to lines 43-63, column 4 of U.S. Pat. No. 3,792,225 which discloses one approach of the prior art.